Device and method for regenerating a train of solitons

ABSTRACT

In the invention, a stream (S) of solitons enters a Sagnac interferometer ( 6 ) with an offset semiconductor amplifier (A) and the amplifier is controlled by an optical synchronization signal (Y) that is amplitude-modulated at a frequency which is half a bit frequency of the stream, the offset of the amplifier in said interferometer and the parameters of said synchronization signal being given values such that two adjacent solitons of said train are transmitted with their optical phase relationship inverted. The invention finds an application in fiber optic telecommunications networks.

BACKGROUND OF THE INVENTION

The present invention concerns a system for regenerating a stream ofsolitons and finds an application in telecommunications networks.

Conveying data to be transmitted over long-haul telecommunications linksin a stream of optical pulses is well known. A stream of this kind istimed, meaning that clock times are defined in regular sequence at apredetermined frequency constituting a “bit frequency”, the data to betransmitted being conveyed in binary form by the presence or absence ofa pulse at each successive clock time. It is well known that the pulsescan advantageously be launched into the fibers that waveguide them inthe form of solitons. It is well known that a soliton is matched to thefiber which guides it and is then characterized by a specific amplitudetime profile, a very short duration at half-height, and high spectralpurity. Because chromatic dispersion and non-linearity effects (Kerreffect) specific to the fiber are compensated, a soliton has theadvantage of propagating in the fiber without cumulative deformation, atleast theoretically. However, the compensation requires that the averagepower of the pulse be maintained as it propagates, but power losses areinevitable and must therefore be compensated by means of a succession ofoptical amplifiers along the link. The amplifiers are typicallyoptically pumped erbium-doped fibers. This need to compensate powerlosses of the pulses gives rise to two unwanted phenomena associatedwith spontaneous emission in the amplifiers: one is Gordon-Haus jitter,which consists in each soliton within the stream of solitons beingdisplaced randomly on either side of the corresponding clock time. Theother is accumulated noise. These two unwanted phenomena can degradequality, in other words they can increase transmission error rate.

One prior art method for avoiding those drawbacks consists insynchronously modulating an optical pulse stream, more especially astream of solitons. The modulation regenerates the stream of solitons.It restores the correct time profile and position of the solitons andeliminates or at least reduces noise. The theory of the method isdescribed in an article by H. Kubota and Nakazawa, “Soliton transmissioncontrol in time and frequency domains”, IEEE J. Quantum Electronics v.29, no. 7, pp. 2189-2197, July 1993. A 10 Gbit/s transmission experimentover one million kilometers is described in an article by Nakazawa etal. (1991), “Experimental demonstration of soliton data transmissionover unlimited distance with soliton control in time and frequencydomains”, Electronics Letters, v. 29, no. 9, pp. 729-730, Apr. 29, 1993.

A system for regenerating a stream of light pulses is described in anarticle by J. K. Lucek and K. Smith (1993) “All optical signalregenerator”, Opt. Lett. V. 18, no. 15, pp. 1226-1228, Aug. 1, 1993.Also worthy of mention is a communication by D. Sandel et al.:“Polarization-independent regenerator with nonlinear optoelectronicphase-locked loop”, Optical Fiber Conference Proceedings 1994, page FG2.

An article by M. Eiselt, W. Piper and H. G. Weber “SLALOM: SemiconductorLaser Amplifier in a Loop Mirror”, Journal of Lightwave Technology, vol.13, no. 10, October 1995, pp. 2099-2112 describes a so-called SLALOMdevice. It indicates in particular that the device can be used forsynchronous regeneration, as described in the section entitled: “SLALOMas Optical Retiming Device”. It does not specifically describe theessential components of a regenerator, i.e. a regeneration system, butrather those of an experimental system for assessing the possibilitiesof a regenerator. A regenerator as described in the above article isreferred to hereinafter as a “prior art SLALOM regenerator”.

Some of the essential. features of a prior art regenerator of the abovekind have analogies with features of a system of the present invention.These features are:

An optical waveguide between an input for receiving a data signalconsisting of a stream of pulses to be regenerated and an output atwhich a stream of regenerated pulses carrying the same data is supplied.The pulses of the stream to be regenerated have an input wavelength. Thestream has a clock rate defining successive clock times at a bitfrequency. A segment of this waveguide constitutes an interferometerloop. The loop is closed by a loop coupler which couples the waveguideto itself at both ends of the loop to constitute a Sagnacinterferometer.

A loop amplifier consisting of a semiconductor laser amplifier connectedin series into the interferometer loop at a distance from the mid-pointof the loop. This distance along the waveguide is referred tohereinafter as the “offset distance”.

Finally, a clock source which supplies an optical signal defining clocktimes corresponding to the stream of pulses to be regenerated. Thesignal is injected into the interferometer loop to bring about thereincross modulation between it and the pulses of the stream. Its wavelengthis referred to as the clock wavelength.

In the prior art SLALOM regenerator, the clock source supplies thesignal mentioned above in the form of clock pulses. The pulses aresupplied to the input of the waveguide of the interferometer loop. Theloop coupler derives components from the data signal pulses and theclock pulses. They circulate in the interferometer loop in two oppositedirections. Those derived from the data signal cause temporarysaturation of the loop amplifier. The offset distance mentioned above ischosen so that the pulses derived from the clock pulses reach theamplifier when it is in a saturated state or a non-saturated statedepending on their time position relative to the previous pulses. Ifboth components derived from a clock pulse reach the amplifier in twodifferent saturation states, they interfere positively in the loopcoupler and the clock pulse is therefore transmitted to the output ofthe waveguide of the interferometer loop. This situation arises if thedata signal includes a pulse in a suitable time position relative to theclock pulse, i.e. if the data signal has opened a time window for theclock pulse. The system therefore behaves like a gate controlled by thedata signal. In this way the data is transferred to the pulse streamconsisting of the clock pulses transmitted to the output of thewaveguide. The new data signal is therefore supplied at the clockwavelength.

It would also appear that in the above prior art regenerator the offsetdistance is defined as a function of the duration of the loop amplifiersaturation state caused by each pulse of the signal initially carryingthe data. This distance is therefore defined as a function of thelifetime of charge carriers in the amplifier.

The prior art SLALOM regenerator has the advantage of being of the“all-optical” kind, which avoids bandwidth limitations associated withthe use of electronic signals. It would appear to have various otheradvantages compared to prior art modulators of other all-optical types.These advantages are in particular that its interferometer loop is muchshorter than those of NOLM type systems and it does not require the useof polarization-maintaining fibers if the loop comprises an opticalfiber and the modulator must be insensitive to polarization. Theseadvantages are important because it is sometimes highly desirable toimplement the modulator in integrated form and because an optical signalhas random polarization when it travels great distances along a fiberoptic link. However, the prior art SLALOM regenerator seems to have theparticular disadvantage that its operation appears to be highlydependent on operating parameters that define the duration of the loopamplifier saturation state. Moreover, if it were applied to regeneratinga stream of solitons, the synchronization signal would itself have to bein the form of a stream of solitons which could be described as “clocksolitons”. Finally, it is necessary to allow for the fact that thewavelength of the new data signal is different from that of the signalreceived at the input. This would be a serious drawback in manyapplications.

SUMMARY OF THE INVENTION

The present invention has the following objects:

to make a SLALOM regenerator relatively insensitive to variations in theoperating parameters of the semiconductor loop amplifier of theregenerator,

to retain the original optical wavelength of the data is signal,

more generally, to retain the advantages of the prior art SLALOMregenerator but avoid its disadvantages, and

to minimize the error rate of a fiber optic transmission system in whichdata is carried by streams of solitons.

With these objects in view, a system in accordance with the invention ischaracterized in particular in that the offset distance of the loopamplifier of the system is not less than a minimum value equal to 20% ofa nominal offset LN such that

LN=c/2B

The offset distance also differs by at least this minimum value of theoffset distance from illegal values equal to the product of the nominaloffset multiplied by an even integer, c being the speed of light in saidwaveguide and B being the bit frequency defined by said clock rate ofthe stream of solitons. The signal representative of the clock times ofthe data signal is a synchronization signal whose optical envelope has afrequency equal to half the bit frequency. Finally, the amplitude andthe phase of the envelope of the synchronization signal and an averagevalue of the power of the signal are such that two solitons reaching theinput of the system and occupying two immediately successive clock timesin a stream of solitons are transmitted to the output of the system inoptical antiphase.

BRIEF DESCRIPTION OF THE DRAWINGS

How the present invention can be put into effect is describedhereinafter by way of non-limiting example and with the aid of theaccompanying diagrammatic drawing.

The drawing shows a system in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The system comprises the following essential components:

A light waveguide G extends between an input 2 for receiving a string Sof solitons to be regenerated and an output 4 at which a stream ofregenerated solitons is supplied. The waveguide may be an optical fiberor it may be formed on a substrate if the system is implemented inintegrated form. The wavelength of the solitons constitutes an inputwavelength. The stream has a clock rate defining successive clock timesat a bit frequency. A segment of the waveguide constitutes aninterferometer loop 6. The loop is closed by a loop coupler 8 couplingthe waveguide to itself at both ends of the loop to constitute a Sagnacinterferometer. The operation of an interferometer of this kind is wellknown in itself, and in outline it is as follows: If an input lightpulse PE is received at the input 2, the coupler 8 splits it into twocomponents DP and RP propagating in the loop in two opposite directions.The coupler then applies a first phase-shift of π/2 to the component RPrelative to the pulse PE. When this component has completed the loop,the coupler applies a second phase-shift of π/2 thereto beforetransmitting it to the output 4, where it interferes with the componentDP that propagated in the waveguide without being subjected to anyphase-shift in the coupler 8. The interference is constructive ordestructive depending on any other phase-shifts to which thesecomponents have been subjected in their respective circuits of the loop6. To be more precise, the constructive or destructive nature of theinterference results from the difference between two respective totalvalues of these other phase-shifts for the two components concerned. Asa result, the pulse PE is transmitted or not to the output 4 accordingto the value of a signal controlling that difference.

A semiconductor laser loop amplifier A is connected in series into theinterferometer loop at a particular distance from a mid-point 10 of theloop. The mid-point is the point at the same distance from both ends ofthe loop. The distance L along the waveguide between the loop amplifierand the mid-point constitutes the “offset distance”. According to thepresent invention, the offset distance L is at least equal to theabove-defined minimum value and differs from said illegal values by atleast this minimum value.

Finally, a synchronization source 12 supplies an optical synchronizationsignal Y which has a synchronization wavelength and whose power varieswith time in a manner that defines the clock times H1 . . . H4 of thestream S of solitons. The synchronization signal Y is injected into theloop amplifier A, where it brings about cross modulation between thesolitons and the signal, by a wavelength-selective synchronizationcoupler 14.

According to the invention, the optical envelope of the synchronizationsignal has a frequency equal to half the bit frequency of the stream ofsolitons to be regenerated. Moreover, the amplitude and the phase of theenvelope and an average value of the power of the signal are such thattwo solitons reaching the input 2 and occupying two immediatelysuccessive clock times in the stream S of solitons (such as H3 and H4)are transmitted to the output 4 with their optical phase relationshipinverted, i.e. the two solitons are transmitted in antiphase if theywere in phase at the input of the system.

This antiphase relationship of two successive optical pulses is referredto in an article by O. G. Okhotnikov and F. M. Araujo, “Pulse generationthrough optical switching in phase driven loop mirror”, ElectronicsLetters 7^(th) December 1995 Vol. 31 No. 25. The article describes anoptical pulse generator using an interferometer loop constituting aSagnac interferometer. The amplifier of the loop is in an offsetposition and is controlled by an electrical signal whose frequency ishalf the repetition frequency of the pulses generated. The pulses arealternately bright pulses and dark pulses because of the alternation oftheir optical phases. The offset distances referred to are differentfrom those of the present invention.

In the context of the present invention, the successive solitons couldbe referred to “adjacent solitons”. Their optical antiphase relationshiphas the advantage of greatly limiting the risk of transmission qualitybeing degraded by soliton collision phenomena. This advantage of anantiphase relationship between adjacent solitons is disclosed in anarticle by Pierre-Luc Francois and Thierry Georges, entitled “Reductionof averaged soliton interaction forces by amplitude modulation”, OpticsLetters, Apr. 15, 1993, Vol. 18, No. 8, pp. 583-585.

It exists independently of time jitter or noise in the stream ofsolitons to be regenerated, with the result that the system of theinvention can be used with advantage even if the stream of solitons tobe regenerated is not affected by jitter or noise. This advantage is notlost after a second stage of synchronous regeneration, which is contraryto what might be expected from a superficial analysis.

The new features (i.e. the offset distance and the frequency of theenvelope of the synchronization signal) not only achieve effectivesynchronous regeneration but also yield the advantage resulting from theantiphase relationship of adjacent solitons. The present inventionteaches that, when the new features are used, there are values of theamplitude, phase, and average power of the synchronization signal suchthat the antiphase relationship is obtained. These latter values will beapparent to the skilled person, on the basis of the above disclosure,and in particular on the basis of the previously mentioned articledescribing the SLALOM devices, over and above which no more thanstandard trial and error is required.

In the context of this invention, the peak-to-peak amplitude of thesynchronization signal is much greater than what would be necessary foran input light pulse having the input wavelength and received at theinput 2, in order to be transmitted or not transmitted to the output 4depending on the time relationship between that light pulse and themodulation of the synchronization signal. This argument can be refinedby considering the phase difference induced in the loop amplifier Abetween two components DP and RP of the same light pulse, the twocomponents traveling round the interferometer loop in a forwarddirection and a retrograde direction, respectively. The phase differenceis related to the variation in the power of the synchronization signalbetween the two times at which the respective components pass throughthe amplifier, or at least between two periods respectively precedingthese two times, the duration of the two periods being defined by thelifetime of the charge carriers in the amplifier.

In the context of this invention, the values of the phase differencecorresponding to two successive clock times are substantiallysymmetrical about the value corresponding to a median time between thetwo clock times. For the system to achieve synchronous regeneration ofthe solitons, the value of the phase difference corresponding to amedian time must prevent an input pulse from being transmitted to theoutput of the system. As a result the amplitude of the modulation of thesynchronization signal is such that the difference between the extremevalues of the phase difference is substantially twice the differencethat would enable the light pulse in question to be transmitted oreliminated, depending on its time relationship with the synchronizationsignal.

The phase variations that can be obtained in prior art amplifierssuitable for use as the loop amplifier are limited in practice, inparticular when the power of the synchronization signal causing thevariations is itself limited. This is why, in the context of thisinvention, it is desirable for the offset distance to have at leastapproximately optimum values enabling the objects of the invention to beachieved with the smallest possible values for the phase variations.This is why the offset distance L is preferably in the range from 80% to120% of said nominal offset LN, possibly increased by the product of thenominal offset by an even integer. This can be expressed in the form

0.8<L/LN−2k<1.2

where k is an integer. Optimum offset values L are approximately givenby the expression: (2k+1) LN. The illegal values for this distance aregiven by the expression: 2k.LN. These illegal values are those for whichthe system could not operate regardless of the characteristics of theloop amplifier and the synchronization signal. The expressions givingthese optimum illegal values become exact in the situation where themodulation of the synchronization signal causes the variation of thephase-shift introduced by the loop amplifier to be sinusoidal.

In one typical embodiment the modulation of the synchronization signal Yis substantially sinusoidal.

The system preferably further includes a wavelength filter F forselectively transmitting to the output 4 light at the input wavelength.

The synchronization source 12 is preferably controlled by the steam S ofsolitons by way of a clock recovery coupler 16 and a waveguide 18 thatconstitute a clock recovery unit. This unit is provided with phasematching means 20 to impart to the synchronization signal a phasesuitable for the stream of solitons.

The synchronization source 12 can include a clock recovery systemsimilar to that described in an article by S. Kawanishi and M.Saruwatari, entitled “New-type phase-locked loop using traveling-wavelaser-diode optical amplifier for very high-speed optical transmission”,Electronics Letters 10^(th) November 1988 Vo.24 No. 23, pp. 1452-1453.It can also include a system for imparting a sinusoidal shape and therequired frequency to the optical envelope of the signal obtained byclock recovery. This latter system can be similar to that described inan article by N. Onodera entitled “THz optical beat frequency generationby modelocked semiconductor lasers”, Electronics Letters May 23, 1966Vol. 32 No. 11, pp. 1013-1014.

To give a numerical example, the bit frequency b can be in the order 5Ghz, the interferometer loop length can be in the range from 5 cm to 100m, approximately, for example around 5 m, and the time offset induced bythe offset distance between the times at which the two components of apulse reach the loop amplifier can be given by the expression 2nL/c=35ps, for example, n being the refractive index seen by these componentsin the fiber constituting the interferometer loop.

What is claimed is:
 1. A system for regenerating a stream of solitons,the system including: an optical waveguide (G) between an input (2) forreceiving a stream (S) of solitons to be regenerated and an output (4)at which a stream of regenerated solitons is supplied, the stream ofsolitons having an input wavelength and a clock rate defining successiveclock times of the stream at a bit frequency B of the stream, a segmentof the waveguide constituting an interferometer loop (6), the loop beingclosed by a loop coupler (8) coupling the waveguide to itself at twoends of the loop to constitute a Sagnac interferometer, a mid-point (10)of the loop being at equal distances along the waveguide from the twoends, and a loop amplifier (A) constituted by a semiconductor loopamplifier connected in series in said interferometer loop, the distancealong the waveguide between the loop amplifier and the mid-pointconstituting an offset distance (L), said offset distance (L) being notless than a minimum value which is 20% of a nominal offset LN such thatLN=c/2B  the offset distance differing by at least this minimum valuefrom illegal values equal to the product of the nominal offsetmultiplied by an even integer, c being the speed of light in saidwaveguide, a synchronization source (12) supplying an opticalsynchronization signal (Y) defining clock times (H1 . . . H4) for saidstream S of solitons and having a synchronization wavelength and a powervarying in accordance with an optical envelope of the signal, theenvelope having a frequency equal to half said bit frequency, and means(14) for injecting said synchronization signal (Y) into said loopamplifier (A) to bring about therein cross modulation between saidsolitons and the synchronization signal, the amplitude and the phase ofsaid envelope of the synchronization signal and an average value of thepower of the signal being such that two solitons reaching said input (2)and occupying two immediately successive clock times (H3, H4) of saidstream (S) of solitons are transmitted to said output (4) with theiroptical phase relationship inverted.
 2. A system according to claim 1,the system being characterized in that said offset distance (L) is inthe range from 80% to 120% of said nominal offset (LN), possiblyincreased by the product of the nominal offset multiplied by an eveninteger.
 3. A system according to claim 1, the system beingcharacterized in that said modulation of the synchronization signal (Y)is substantially sinusoidal.
 4. A system according to claim 1, thesystem further including a wavelength filter (F) for selectivelytransmitting light at said input wavelength to said output (4).
 5. Asystem according to claim 1, said synchronization source (12) beingcontrolled by said stream of solitons through the intermediary of aclock recovery coupler (16) and a waveguide (18) constituting a clockrecovery unit which has phase matching means (20) to impart to saidsynchronization signal a phase suitable for said stream of solitons. 6.A system according to claim 1, said stream of solitons constituting adata signal.
 7. A method of regenerating a stream of solitons, in whichmethod a stream (S) of solitons enters a Sagnac interferometer (6) withan offset semiconductor amplifier (A) and the amplifier is controlled byan optical synchronization signal (Y) amplitude-modulated at a frequencywhich is half a bit frequency of the stream, the offset of the amplifierin said interferometer and the parameters of said synchronization signalbeing such that two adjacent solitons of said stream are transmittedwith their optical phase relationship inverted.